Entry board for drilling small holes, a method of making the same and a method for drilling small holes through a printed circuit board by using said entry board

ABSTRACT

An entry board for drilling small holes having a front surface layer made of aluminum or aluminum alloy to be disposed at the drilling inlet side and a back surface layer made of aluminum or aluminum alloy to be disposed at the drilling outlet side. The back surface layer is adhered or joined by clad rolling to said front surface layer. The hardness of the front surface layer is smaller than that of the back surface layer.

FIELD OF THE INVENTION

The present invention relates to an entry board for drilling smallholes, e.g. having a diameter of 0.1 mm-2.0 mm, through an articlerequiring high accuracy such as a printed circuit board, and alsorelates to a method of making the entry board and further relates to amethod for drilling small holes through a printed circuit board by usingthe entry board.

BACKGROUND OF THE INVENTION

Electronic techniques have been widely applied in various kinds offields including not only the electric or communication fields oftelephones, televisions, computers (including personal computers),lights and controlling devices utilized in various kinds ofmanufacturing industries, but also other fields including automobiles,office electronic supplies, cameras, toys, and the like. At the sametime, the techniques have required high performance and high accuracy.Thus, in the fields of printed circuit boards, circuit patterns formedon the board have rapidly become more minute, as well as high densityand multi-layered. As a result, the width of each circuit pattern andthe distance therebetween have become narrower. Especially, in printedcircuit boards of a through hole type, the holes have become smaller indiameter, and the number of holes has been increased. Further, thepositions of holes have required high accuracy.

In drilling holes through a multilayered board, drilling small holeseach having a diameter of 0.3-0.4 mm has been conventionally conducted.Recently, drilling much smaller holes each having a diameter of 0.1-0.3mm has been practically conducted, and such demands as drilling smallerholes and increasing the number of holes with higher accuracy ofdrilling position will be further required.

In drilling such small holes, as shown in FIG. 4, it has been known thatplacing an entry board 1 on a printed circuit board 5 enables easyintroduction of a drill 8 which in turn improves accuracy of thedrilling positions of a hole 9 drilled on the is printed circuit board 5and also enables drilling holes without causing burrs, or the like,around the holes. There have been several proposals concerning suchentry boards.

Japanese Patent Publication 61-61921 (hereinafter referred to as JP61-61921) discloses a method for drilling through a printed circuitboard by using an entry board. The entry board comprises a base platemade of wood pulp and glass fiber and aluminum foils or the likecovering both surfaces of the base plate. Japanese Patent UnexaminedPublication SHO 62-214000 (hereinafter referred to as JP 62-214000)discloses a method for drilling a printed circuit board by using anentry board which comprises a base plate made of aluminum foils andpapers covering both surfaces of the base plate. Japanese PatentUnexamined Publication SHO 63-11207 (hereinafter referred to as JP63-11207) discloses a method for drilling through a printed circuitboard by placing an entry board made of metallic foil on the printedcircuit board and by way of an adhesive layer formed on one surface ofthe metallic foil.

However, in the method disclosed in JP 61-61921, because hard materialssuch as JIS (Japanese Industrial Standard) 3003-H19 homogenized aluminumalloy as the aluminum foils are used as a surface layer of an entryboard, a drill tends to slip on the aluminum foil, resulting indecreased drill positioning accuracy. Further, in such a case thatadhesive layers are used to connect the aluminum foils to the baseplate, the entry board becomes a five-layered structure, resulting in anincreased manufacturing cost. In the disclosure of JP 62-214000, thepaper thereof lacks heat-resistance ability and paper dust is generatedduring drilling, resulting in problems in the following procedures suchas a plating procedure. Further, in the method disclosed in JP 6311207,it is unavoidable to have the sticking of chips produced when drillingto the adhesive layers.

As mentioned above, in the conventional methods, there have still beenproblems in drilling small holes. Further, as the pattern formed on theprinted circuit board reduces in size, the size of each hole to bedrilled becomes smaller and the number of the holes increases, therotation speed of the drill becomes higher and the diameter of the drillbecomes smaller which results in rotational deflection of the drill.Furthermore, there have been more strict demands for improving accuracyof the drilling position and decreasing the allowable height of burrs.In spite of such demands, as the above mentioned conventional entryboards are still being used, accuracy of the drilling position has notbeen improved and drill breakages occur when it is attempted to improvethe accuracy of the drilling position and to decrease the generation ofburrs.

SUMMARY OF THE INVENTION

In view of the foregoing disadvantages that are inherent in conventionalentry boards, the present invention aims to provide an improved entryboard for drilling small holes through a printed circuit board, or thelike, which enables easy introducing of a drill, improving accuracy ofdrilling position by restraining slipping of a drill on the surface ofthe entry board, increasing the number of work items ("works") to bedrilled such as a printed circuits board at one time, decreasing thenumber of drill breakages and further decreasing the generation ofburrs.

It is also an object of the invention to provide a method for making theentry board mentioned above.

It is another object of the invention to provide a method of drillingsmall holes by using the entry board mentioned above.

The above and other objects are accomplished by an entry board includinga front surface layer made of aluminum or aluminum alloy disposed on adrilling inlet side, a back surface layer made of aluminum or aluminumalloy disposed on a drilling outlet side, wherein both the layers arebonded together or are joined by clad rolling, and wherein hardness ofthe front surface layer is smaller than that of the back surface layer.

Because of the front surface layer having smaller hardness, a drill caneasily be introduced, accuracy of drilling position can be improved andslipping of a drill on the entry board can be restrained, vibration of arotating drill caused by a thin drill can be effectively restrained anddrill breakages can be decreased. On the other hand, because of the backsurface layer having higher hardness, generation of burrs can bedecreased.

Thickness of the front surface layer may preferably fall within therange of from 5 to 100 μm and micro vickers hardness (Hv) of the frontsurface layer may fall within the range of from 20-50. Thickness of theback surface layer may preferably fall within the range of from 30-200μm and micro vickers hardness (Hv) of the back surface layer may exceed50, but does not exceed 140.

Roughening treatment may preferably be conducted on the surface of thefront layer to obtain a central average roughness (Ra) of 0.15-0.30 μm.This treatment is effective to prevent a drill from slipping whendrilling and improves accuracy of drilling position. However, hardnessof the front surface layer most effectively contributes to improvementof the accuracy of the drilling position. The next importantcontributions to such improvement include thickness and surfaceroughness.

The above objects are also accomplished by a method for making an entryboard. The method includes the steps of clad rolling a pure aluminumalloy sheet to be composed of a surface layer and an Al--Mn aluminumalloy to be composed of a back surface layer, and then annealing them atthe temperature of 200-260° C. This method easily enables one todifferentiate hardness of both of the layers by utilizing softeningcharacteristics thereof.

Furthermore, the above objects are accomplished by a method for drillingsmall holes through a printed circuit board. The method includes thesteps of preparing an entry board. The entry board includes a frontsurface layer made of aluminum or an aluminum alloy to be disposed on adrilling inlet side and a back surface layer made of aluminum or analuminum alloy to be disposed on a drilling outlet side. The frontsurface layer is glued together by an adhesive layer or joined by cladrolling, and hardness of the front surface layer is smaller than that ofthe back surface layer. The method also includes the steps of placingthe entry board on piled printed circuit boards such that the backsurface layer contacts a printed circuit board, and drilling through theprinted circuit boards together with the entry board. This methodenhances productivity of drilling with high accuracy.

The above and other objects and features of the present invention willbe apparent from the following detailed description of the inventionwith reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an enlarged cross-sectional view of an entry board fordrilling holes according to the first embodiment of the presentinvention.

FIG. 2 is an enlarged cross-sectional view of an entry board fordrilling small holes according to the second embodiment of the presentinvention.

FIG. 3 is a graph showing the relationship between hardness (Hv) andannealing temperature (C°) of the front and back surface layersaccording to the second embodiment.

FIG. 4 is a schematic view showing drilling small holes through aprinted circuit board by using an entry board.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be described, indetail, with reference to the accompanying drawings.

An entry board according to the present invention is made of aluminum oraluminum alloy. An entry board 1 according to the first embodiment shownin FIG. 1 includes a front surface layer 2 made of aluminum or aluminumalloy and a back surface layer 4 made of aluminum or aluminum alloy.Both of the layers 2 and 4 are glued together by way of an adhesivelayer 3. On the other hand, an entry board 1 according to the secondembodiment shown in FIG. 2 is a two-layer structure joined by cladrolling.

The front surface layer 2 is to be disposed at a drilling inlet side andthe back surface layer 4 is to be disposed at a drilling outlet side.Hardness of the front surface layer 2 should be smaller than that of theback surface layer 4. The reasons are as follows.

Though it is effective to reduce height of burrs if the hardness of anentry board is high, the drill tends to slip when a drill contacts thesurface of the entry board, thereby decreasing the accuracy of thedrilling position. On the other hand, though it is easy to keep theaccuracy of centering the drill if the hardness of an entry board islow, it is inevitable to increase the generation of burrs.

In the present invention, to accomplish these opposite demands, i.e., ademand for improving accuracy of drilling position by preventingslipping of a drill and a demand for decreasing the height of burrs, asa front surface layer 2 to which a drill contacts at first when drillingsoft materials are used to prevent slipping of a drill, and as a backsurface layer 4 hard layer materials having a hardness higher than thefront surface layer 2 are used to prevent generating burrs.

As a result, demands, such as the improvement of the accuracy of thedrilling position on a printed circuit board whose pattern formedthereon has rapidly reduced in size, the minimization of the height ofburrs, and the reduction of drill breakages, have been satisfied. Sincethe height of a burr becomes lower, the number of printed circuit boardsto be drilled at one time can be increased, thereby enhancing drillingproductivity. Further, decreasing the height of burrs is effective forpreventing the causing of damages to circuits made of copper foils on aprinted circuit board. Thus, an improved entry board and a method fordrilling small holes through a printed circuit board by using the entryboard has been successfully developed.

Thickness of the front surface layer 2 should be determined depending ona diameter of a drill to be used. Too thick of a front surface layerrelative to the diameter of the drill causes clinging on the drill dueto its soft layer. Thus, the thickness should be determined depending ona diameter of a drill to be used. Generally, thickness of the frontsurface layers 2 falls within the range of from 5 to 100 μm, andpreferably from 5 to 50 μm where a diameter of a drill is not largerthan 0.5 mm. The micro vickers hardness (Hv) of the surface layer 2falls within the range of from 20 to 50, more preferably from 25 to 40.It is necessary to soften materials of the surface layer so as to easilyintroduce the drill. Easy introducing of the drill can prevent the drillfrom slipping and can improve accuracy of drilling position.

As for the back surface layer 4, the thickness thereof may preferablyfall within the range of from 30 to 200 μm. Micro vickers hardness (Hv)of the layer may preferably exceed 50, but does not exceed 140. Morepreferably, in the adhesive type shown in FIG. 1, micro vickers hardness(Hv) falls within the range of from 65 to 140, and in the clad rolledtype shown in FIG. 2. from 58 to 140. The back surface layer mentionedabove can decrease the height of burrs compared to that of burrsgenerated during conventional drilling procedures or can at leastrestrict the height of burrs to the same level of that of burrsgenerated during conventional drilling procedures.

The preferable total thickness of the entry board 1 varies depending onthe diameter of the drill 8 to be used. The thickness of the entry board1 should be thick where the diameter of the drill is large. On thecontrary, the thickness of the entry board 1 should be thin where thediameter of the drill is small. Where the diameter of the drill is notmore than 0.5 mm, the total thickness of the entry board 1 maypreferably be 30 to 200 μm, more preferably 50-100 μm. As apparent fromthe above, the thickness of the back surface layer 4 is determined bythe thickness of the front surface of the layer 2.

The surface roughness of the aluminum foil used as an entry board whichis now being used is generally 0.1 μm or the like in the central averageroughness (Ra) and causes no problems so far. However, in a case thatthe drilling of small holes through a printed circuit board with ahigher performance is required, even if the surface materials of theentry board to which a drill contacts first is soft, such as microvickers hardness of 20-50, the entry board having such a surface of highflatness may cause slipping of the drill when the drill contacts thesurface, thereby lowering the accuracy of the drilling position. Toprevent slipping, the surface of the front surface layer 2 of the entryboard 1 is preferably roughened so that the central average roughness(Ra) falls within the range of from 0.15-0.30 μm. This can preventslipping of the drill and can improve the accuracy of the drillingposition.

When the central average roughness (Ra) of the surface is less than 0.15μm, slipping of a drill cannot be effectively prevented. On thecontrary, when the roughness exceeds 0.30 μm, a burden to be loaded tothe drill when it contacts the entry board at first becomes large andmay cause drill breakages.

When drilling through a plurality of printed circuit boards stacked oneon another, if the drill is inserted in an inclined state, largedrilling position errors can occur at the lowermost printed circuitboard. In drilling small holes, a drilling position error of 5 μm isserious. Thus, it is important to roughen the surface of the entryboard.

Roughening the surface of the front surface layer 2 enables one toeasily distinguish the front surface layer 2 from the back surface layer4 in actual use so as to avoid mistakes.

The entry board according to the first embodiment shown in FIG. 1 isproduced by bonding the front surface layer 2 and the back surface layer4, each made of aluminum or aluminum alloy foil, by way of an adhesivelayer 3. As for the materials of the front surface layer 2 and the backsurface layer 4, the compositions are not limited so long as theirhardness fall within the ranges described above. Pure aluminum or otheraluminum alloys can be used. JIS 1085, 1N30, 1050, 1100 or 3003 aluminumis preferably used. JIS 1085, 1N30 and 3003 aluminum are more preferablyused because they are excellent in rolling performance and are generallyused. An Al--Si aluminum alloy is not recommended to be used asmaterials of the layers because the alloy includes Si which causes heavyabrasion to the drill. Under the condition that the front surface layer2 is disposed at a drilling inlet side and the back surface layer 4 isdisposed at a drilling outlet side, each layer 2 or 4 is not limited toone sheet of an aluminum or an aluminum alloy layer, but may include aplurality of laminated layers.

As for the adhesive layer 3, adhesive materials which do not causeclinging to the drill by heat generated during drilling, such asthermosetting acryl, polyamide, rubber, epoxy, urethane adhesive, may beused. Urethane adhesive is preferably used in light of its highperformance and low cost. Though the thickness of the adhesive layer isnot limited, the amount of the adhesive layer generally falls within therange of from 1-5 g/m². The adhesive layer thicker than the aboveincreases the cost.

Preferable methods for producing the entry board 1 according to thesecond embodiment shown in FIG. 2 will now be described below.

First, materials to be a front surface layer 2 and a back surface layer4 are prepared. The components of the materials of the front surfacelayer 2 and the back surface layer 4 are not limited where the hardnessfalls within the range mentioned above. Pure aluminum or the otheraluminum alloys may be used. However, as will be mentioned below, inorder to be able to differentiate the hardness of the front surfacelayer 2 and the back surface layer 4 by annealing which will beperformed after joining the front surface layer 2 and the back surfacelayer 4 by clad rolling, pure aluminum alloys such as JIS 1N30, 1050,1100 may be used as the front surface layer 2 and Al--Mn alloys such asJIS 3003, 3004 or Al--Mn--Mg alloy such as JIS 5182 may be used as theback surface layer 4. An Al--Si alloy is not recommended to be used asmaterials of the layers because the alloy includes Si which causes heavyabrasion to the drill. Under the condition that the front surface layer2 is disposed on the drilling inlet side and the back surface layer 4 isdisposed on the drilling outlet side, each layer 2 or 4 is not limitedto one sheet of an aluminum layer but may include a plurality of claddedlayers.

Then, the above-mentioned materials for a front surface layer 2 and aback surface layer 4 are placed one on another and then clad rolled. Theclad rolling may be conducted under the normal conditions. Preferably,as mentioned above, the rolling procedure is conducted such that thefront surface layer 2 becomes 5-100 μm in thickness and the back surfacelayer 4 becomes 30-200 μm in thickness and the obtained entry boardmaterials are wound to be a coil.

Then, the entry board materials are annealed. As shown in the graph inFIG. 3 showing softening performance, a pure aluminum alloy comprisingthe front surface layer 2 and Al--Mn alloy or Al--Mn--Mg alloycomprising the back surface layer 4 are different in the softeningperformance. By utilizing this characteristic, a front surface layer 2and a back surface layer 4 which are different in hardness can beobtained by controlling the temperature conditions. Concretely, theannealing is conducted at the temperature of 200-260° C. By setting thetemperature mentioned above, as the softening the front surface layer 2is enhanced and the softening the back surface layer 4 is delayed, apredetermined difference in hardness between both layers is generated.Thus, the front surface layer 2 becomes softer than the back surfacelayer 4. If the annealing temperature is lower than 200° C., softeningof both of the layers will not be performed. Thus, it is difficult todifferentiate the hardness between them. On the other hand, if thetemperature exceeds 260° C., softening of the back surface layer 4 isalso performed. Thus, it is also difficult to differentiate the hardnessbetween them.

The annealing may be a batch annealing in which annealing is conductedin a coiled state, or may be a continuous annealing in which annealingis continuously conducted while uncoiling. However, the continuousannealing is preferable because of the following merits. The materialspositioned at the surface side of the coil and the materials positionedat the innermost side of the coil can be annealed at an eventemperature. The required time for annealing is short. When an aluminumalloy including Mg is used as materials, in the batch annealing, it isrequired to conduct it in an inert gaseous atmosphere so as not to causeprecipitation of Mg which looks white. However, in the continuousannealing, the annealing will be accomplished before causingprecipitation of Mg, because the required annealing time is short. Thus,the aluminum alloy does not become white. In a continuous annealing, theannealing time may preferably be 5-60 seconds because less than 5seconds is not enough time to anneal them and exceeding 60 seconds maycause softening of the back surface layer 4. In a batch annealing, theannealing time may preferably be 10-30 hours because not more than 10hours is not enough time to anneal them but exceeding 30 hours may causesoftening of the back surface layer 4.

In order to roughen the surface of the front surface layer 2 such thatthe central average roughness (Ra) is 0.15-0.30 μm, rolling may beperformed by using a roll having a rough surface (Ra=0.15-0.30 μm) atthe front surface layer 2 side and a roll having a normal rough surface(Ra=0.1 μm or the like) at the back surface layer 4 side.

The entry board 1 according to the present invention is placed such thatthe back surface layer 4 contacts one or a plurality of stacked printedcircuit boards 5 as shown in FIG. 4 and then drilled by a drill 8. InFIG. 4, the numeral 6 denotes a backup board and the numeral 7 denotes aspindle.

(Embodiment 1)

This embodiment relates to a bonded structural entry board 1 as shown inFIG. 1.

As composing materials of the front surface layer 2, materials denotedas JIS homogenized symbol "O" and "H14" of JIS 1N30 aluminum alloy, andmaterials denoted as JIS homogenized symbol "O" of JIS 3003 aluminumalloy have been prepared. As composing materials of the back surfacelayer 4, materials denoted as JIS homogenized symbol "H18" and "H19" ofJIS 3003, JIS 3004 and JIS 5182 having thickness are shown in Table 1.The above mentioned JIS homogenized symbol "O" stands for the mostsoftened state, and JIS homogenized symbol "H" stands for a hardenedstate.

Each material of the front surface layer 2 and back surface layer 4 werecombined as shown in Table 1 and bonded by way of an adhesive layer toform various kinds of entry boards.

Also, prepared were entry boards comprising a simple substance made ofmaterials denoted as JIS homogenized symbol "H18" of each JIS 1100 andJIS 3003 aluminum alloy as comparisons.

Each entry board prepared above was used to drill through a printedcircuit board and then evaluated. The drilling was performed as follows.The entry board, two pieces of 6 laminated glass-epoxy layer each havingthickness of 1.6 mm, and a backup board made of a paper-phenol laminatedlayer having a thickness of 1.6 mm were piled one on another and weredrilled under the following conditions.

Drilling bit: 0.35 mm width

Rotation Speed: 80000 rpm

Sending speed: 1.6 mm/min.

The hardness and thickness of the front and back surface layers of eachentry board, the central average roughness (Ra) of the front surfacelayer 2, the accuracy of drilling position after drilling and the heightof the burrs are shown in Table 1.

The micro vickers hardness (Hv) was measured by the Dynamic Super MicroHardness Measuring Apparatus of SHIMADZU SEISAKUSYO, LTD. (Model:DUH-201) under the conditions of a load: 5 gf, holding time: 5 secondsand loading speed: 0.3375 gf/second.

                                      TABLE 1                                     __________________________________________________________________________            Front Surface layer  Back Surface Layer                                                                            Evaluation                                        Hard-                Hard-                Height                          Homo-                                                                             ness                                                                             Thick-                                                                            Surface   Homo-                                                                             ness                                                                             Thick-                                                                            Max. drilling of                 Sample  Aluminum                                                                           geni-                                                                             (Hv)                                                                             ness                                                                              roughness                                                                          Aluminum                                                                           geni-                                                                             (Hv)                                                                             ness                                                                              position error                                                                       Drill                                                                                burrkage           No.     Materials                                                                          zation                                                                            .sup.1)                                                                          (μm)                                                                           (Ra:μm)                                                                         Materials                                                                          zation                                                                            .sup.1)                                                                          (μm)                                                                           (μm).sup.2)                                                                       .sup.3)                                                                              .sup.4)            __________________________________________________________________________    EMBODIMENT                                                                    1       1N30 0   26 30  0.10 3004 H19 85 70  45     3000 holes                                                                           4K                 2       1N30 0   26 30  0.15 3004 H19 85 70  35                               .sup.5)                                                                       .sup.5)                                                                       3       1N30 0   26 30  0.19 3004 H19 85 70  35     3000 holes                                                                           4K                 4       1N30 0   26 30  0.28 3004 H19 85 70  40                               .sup.5)                                                                       .sup.5)                                                                       5       1N30 0   26 30  0.34 3004 H19 85 70  55                               .sup.5)                                                                       .sup.5)                                                                       6       1N30 0   26 30  0.10 3003 H18 65 70  45     3000 holes                                                                           5K                 7       1N30 0   26 30  0.19 3003 H18 65 70  35     3000 holes                                                                           5K                 8       1M30 0   26 20  0.10 5182 H18 138                                                                              80  45     3000 holes                                                                           3K                 9       1N30 H14 38 30  0.10 3004 H19 85 70  45     3000 holes                                                                           4K                 10      3003 0   36 30  0.10 3004 H19 85 70  40     3000 holes                                                                           4K                 COMPARISON                                                                    1       3003 H18 65 150 0.095                                                                              Non-clad        70     1700 holes                                                                           6G                 2       1100 H18 55 150 0.09 Non-clad        70     1900 holes                                                                           8G                 __________________________________________________________________________     .sup.1) Hardness: Micro Vickers Hardness (Hv)                                 .sup.2) Maximum drilling position error (μm)                               .sup.3) Drill breakage: 3000 holes OK; No Drill breakage was occurred whe     3000 holes were made. 1700 holes NG; Drill breakage was occurred when 170     holes were made.                                                              .sup.4) Height of burr: Maximum height of burr after 1000 holes were made     (μm).                                                                      .sup.5) Unmeasured                                                       

(Embodiment 2)

This embodiment relates to an entry board 1 joined by clad rolling asshown in FIG. 2.

As composing materials of the front surface layer 2, JIS 1N30 which is apure aluminum alloy was prepared. As composing materials of the backsurface layer 4, JIS 3003 and JIS 3004 of Al--Mn aluminum alloy and JIS5182 of Al--Mn--Mg aluminum alloy were prepared.

Each composing materials of the front and back surface layers 2 and 4was clad-rolled under the normal conditions and annealed under theconditions shown Table 1 to prepare entry boards. The annealing wascontinuously conducted while uncoiling. The surface of the front surfacelayer 2 was roughened by adjusting the surface roughness of the pressroll which contacts the front surface layer 2 during the final stage ofthe rolling procedure.

The above-mentioned entry boards were used to drill through a printedcircuit board under the same conditions applied to the first embodimentand then evaluated.

The materials, homogenization, hardness and thickness of the front andback surface layer of each entry board and the central average roughness(Ra) of the front surface layer 2 are shown in Table 2. The accuracy ofthe drilling position after drilling and the height of the burrs areshown in Table 3.

                                      TABLE 2                                     __________________________________________________________________________            Front Surface Layer   Back Surface Layer                                                Hard-                Hard-  Annealing                                         ness                                                                             Thick-                                                                            Surface   Ho- ness                                                                             Thick-        Tempe-                Sample  Aluminum                                                                           Homoge-                                                                            (Hv)                                                                             ness                                                                              roughness                                                                          Aluminum                                                                           moge-                                                                             (Hv)                                                                             ness          rature                                                                            Time              No.     Materials                                                                          nization                                                                           .sup.6)                                                                          (μm)                                                                           (Ra:μm)                                                                         Materials                                                                          nization                                                                          .sup.6)                                                                          (μm)                                                                           Method    (°C.)                                                                      (sec)             __________________________________________________________________________    EMBODIMENT                                                                    11      1N30 0    26 75  0.10 3003 (H28).sup.7)                                                                      61 75  continuous                                                                              230ealing                                                                         10                12      1N30 0    26 75  0.19 3003 (H28).sup.7)                                                                      61 75  continuous                                                                              230ealing                                                                         10                13      1N30 0    26 75  0.34 3003 (H28).sup.7)                                                                      61 75  continuous                                                                              230ealing                                                                         10                14      1N30 (H24).sup.7)                                                                       40 75  0.10 3003 (H28).sup.7)                                                                      63 75  continuoua                                                                              250ealing                                                                         10                15      1N30 (H24).sup.7)                                                                       40 75  0.19 3003 (H2S)7)                                                                           63 75  continuous                                                                              250ealing                                                                         10                16      1N30 0    26 30  0.10 3003 (H28).sup.7)                                                                      61 70  continuoua                                                                              230ealing                                                                         10                17      1N30 (H24).sup.7)                                                                       40 30  0.19 3003 (H28).sup.7)                                                                      63 70  continuous                                                                              250ealing                                                                         10                18      1N30 0    26 75  0.10 3004 (H28).sup.7)                                                                      81 75  continuous                                                                              230ealing                                                                         10                19      1N30 (H24).sup.7)                                                                       40 30  0.19 3004 (H28).sup.7)                                                                      83 70  continuous                                                                              250ealing                                                                         10                20      1N30 0    26 30  0.10 5182 (H28).sup.7)                                                                      120                                                                              70  continuous                                                                              220ealing                                                                         10                __________________________________________________________________________     .sup.6) Hardness: Micro Vickers Hardness (Hv)                                 .sup.7) Though there is no JIS standard for midhardness in Aluminum foil,     the homogenization corresponds to "H24" and "H28" of Aluminum.           

                  TABLE 3                                                         ______________________________________                                                Evaluation                                                                    Max. drilling position  Height of                                     Sample No.                                                                              error (μm).sup.8)                                                                        Drill breakage.sup.9)                                                                     burr.sup.10)                              ______________________________________                                        EMBODIMENT                                                                    11        35            3000 holes OK                                                                             5                                         12        30            3000 holes OK                                                                             5                                         13        50            3000 holes OK                                                                             5                                         14        45            3000 holes OK                                                                             4                                         15        40            3000 holes OK                                                                             4                                         16        45            3000 holes OK                                                                             5                                         17        50            3000 holes OK                                                                             4                                         18        40            3000 holes OK                                                                             4                                         19        55            3000 holes OK                                                                             4                                         20        55            3000 holes OK                                                                             3                                         ______________________________________                                         .sup.8)Maximum drilling position error (μm)                                .sup.9)Drill breakage: 3000 holes OK; No drill breakage was occurred when     3000 holes were made.                                                         .sup.10)Height of burr: Maximum height of burr after 1000 holes were made     (μm)                                                                  

As understood from Tables 1-3, the entry board according to the presentinvention is effective to minimize maximum hole position errors, heightof burrs and drill breakages.

Since the entry board for drilling small holes according to the presentinvention includes a front surface layer made of aluminum or aluminumalloy to be disposed at a drilling inlet side and a back surface layermade of aluminum or aluminum alloy to be disposed at a drilling outletside, wherein the front surface layer is bonded to the back surfacelayer by way of an adhesive layer or joined by clad rolling, and whereinthe hardness of the front surface layer is smaller than that of the backsurface layer, the following effects can be obtained.

Since the entry board is mainly made of aluminum foil, a drill can bekept clean and heat generated during a drilling procedure can beeffectively diffused.

Since the front surface layer is made of soft materials, a drill caneasily be inserted into the surface of the front surface layer, slippingof a drill can be prevented, accuracy of the drilling position can beimproved, drill breakages can be prevented and vibration of a rotatingdrill, e.g., caused by a thin drill, can be restricted.

Since the back surface layer is made of a harder material, burrs can beprevented from being generated. Thus, it enables one to drill through aplurality of materials to be drilled at one time, thereby enhancing theproductivity. As the height of burrs are restricted as mentioned above,it is effective to prevent the breakage of copper foil forming circuitson a printed circuit board. Thus, reduced pattern circuits can beeffectively prevented from being destroyed.

The above effects can be surely and stably obtained under the conditionthat the thickness of the front surface layer is 5-100 μm, the microvickers hardness (Hv) of the front surface layer is 20-50, the thicknessof the back surface layer is 30-200 μm, and the micro vickers hardness(Hv) of the back surface layer exceeds 50, but does not exceed 140.

If roughening surface treatments are performed such that a centralaverage roughness is 0.15-0.30 μm, slipping of the drill when drillingcan be prevented, thereby further enhancing accuracy of drillingposition. Furthermore, it enables one to easily distinguish the frontsurface layer from the back surface layer in actual use--thus,preventing improper placement of the entry board upside down.

If the method for making an entry board including the steps of cladrolling a pure aluminum alloy to be a front surface layer and an Al--Mnaluminum alloy or Al--Mn--Mg alloy to be a back front surface layer toform a laminated board and thereafter annealing the laminated board atthe temperature of 200-260° C. is employed, the hardness of the frontand back surface layers can be easily differentiated by utilizingsoftening characteristics thereof by annealing in an easily handledlaminated state. Thus, an entry board can be efficiently manufactured bysuch a simple method.

If the entry board mentioned above is used for drilling small holesthrough a printed circuit board, the drilling can be performed with ahigh accuracy.

Although the invention has been described in connection with specificembodiments, the invention is not limited to such embodiments and aswould be apparent to those skilled in the art, various substitutions andmodification within the scope and spirit of the invention arecontemplated.

What is claimed is:
 1. An entry board for drilling small holes,comprising:a front surface layer made of aluminum or aluminum alloy tobe disposed at a drilling inlet side; and a back surface layer made ofaluminum or aluminum alloy to be disposed at a drilling outlet side,said back surface layer being joined to said front surface layer;wherein hardness of said front surface layer is smaller than that ofsaid back surface, and wherein said front surface layer and said backsurface layer are flat and joined without any unjoined portiontherebetween.
 2. An entry board for drilling small holes, comprising:afront surface layer made of aluminum or aluminum alloy to be disposed ata drilling inlet side; and a back surface layer made of aluminum oraluminum alloy to be disposed at a drilling outlet side, said backsurface layer being joined to said front surface layer by clad rolling;wherein hardness of said front surface layer is smaller than that ofsaid back surface layer, and wherein said front surface layer and saidback surface layer are flat and joined without any unjoined portiontherebetween.
 3. The entry board for drilling small holes as recited inclaim 1, wherein micro vickers hardness (Hv) of said front surface layeris 20-50, and wherein micro vickers hardness (Hv) of said back surfacelayer exceeds 50 but does not exceed
 140. 4. The entry board fordrilling small holes as recited in claim 2, wherein micro vickershardness (Hv) of said front surface layer is 20-50, and wherein microvickers hardness (Hv) of said back surface layer exceeds 50 but does notexceed
 140. 5. The entry board for drilling small holes as recited inclaim 3 or 4, wherein the micro vickers hardness (Hv) of said frontsurface layer is 25-40.
 6. The entry board for drilling small holes asrecited in claim 3, wherein the micro vickers hardness (Hv) of said backsurface layer is 65-140.
 7. The entry board for drilling small holes asrecited in claim 4, wherein the micro vickers hardness (Hv) of said backsurface layer is 58-140.
 8. The entry board for drilling small holes asrecited in claim 1, 2, 3, 4, 6 or 7, wherein thickness of said frontsurface layer is 5-100 μm and thickness of said back surface layer is30-200 μm.
 9. A method for making an entry board for drilling smallholes, comprising the steps of:preparing an entry board having a frontsurface layer made of aluminum or aluminum alloy to be disposed at adrilling inlet side and a back surface layer made of aluminum oraluminum alloy to be disposed at a drilling outlet side, said backsurface layer being joined to said front surface layer, wherein hardnessof said front surface layer is smaller than that of said back surfacelayer; placing said entry board on a plurality of stacked printedcircuit boards so that said back surface layer contacts said stackedprinted circuit boards; and drilling said printed circuit boards.
 10. Amethod for making an entry board for drilling small holes, comprisingthe steps of:preparing an entry board having a front surface layer madeof aluminum or aluminum alloy to be disposed at a drilling inlet sideand a back surface layer made of aluminum or aluminum alloy to bedisposed at a drilling outlet side, said back surface layer being joinedto said front surface layer by clad rolling, wherein a hardness of saidfront surface layer is smaller than that of said back surface layer;placing said entry board on a plurality of stacked printed circuitboards so that said back surface layer contacts said stacked printedcircuit boards; and drilling said printed circuit boards.
 11. An entryboard for drilling small holes, comprising:a front surface layer made ofaluminum alloy to be disposed at a drilling inlet side; and a backsurface layer made of aluminum or aluminum alloy to be disposed at adrilling outlet side, said back surface layer being joined to said frontsurface layer; wherein hardness of said front surface layer is smallerthan that of said back surface layer, and wherein a surface of saidfront surface layer is roughened such that central average roughness(Ra) is 0.15-0.30 μm.
 12. An entry board for drilling small holes,comprising:a front surface layer made of aluminum or aluminum alloy tobe disposed at a drilling inlet side; and a back surface layer made ofaluminum or aluminum alloy to be disposed at a drilling outlet side,said back surface layer being joined to said front surface layer by cladrolling; wherein hardness of said front surface layer is smaller thanthat of said back surface layer, and wherein a surface of said frontsurface layer is roughened such that central average roughness (Ra) is0.15-0.30 μm.
 13. The entry board for drilling small holes as recited inclaim 11, wherein micro vickers hardness (Hv) of said front surfacelayer is 20-50, and wherein micro vickers hardness (Hv) of said backsurface layer exceeds 50 but does not exceed
 140. 14. The entry boardfor drilling small holes as recited in claim 12, wherein micro vickershardness (Hv) of said front surface layer is 20-50, and wherein microvickers hardness (Hv) of said back surface layer exceeds 50 but does notexceed
 140. 15. The entry board for drilling small holes as recited inclaim 13 or 14, wherein the micro vickers hardness (Hv) of said frontsurface layer is 25-40.
 16. The entry board for drilling small holes asrecited in claim 13, wherein the micro vickers hardness (Hv) of saidback surface layer is 65-140.
 17. The entry board for drilling smallholes as recited in claim 14, wherein the micro vickers hardness (Hv) ofsaid back surface layer is 58-140.
 18. The entry board for drillingsmall holes as recited in claim 11, 12, 13, 14, 15, 16 or 17, whereinthickness of said front surface layer is 5-100 μm and thickness of saidback surface layer is 30-200 μm.
 19. The entry board for drilling smallholes as recited in claim 15, wherein thickness of said front surfacelayer is 5-100 μm and thickness of said back surface layer is 30-200 μm.